Zinc dithiocarbamate-containing lubricant oil compositions are generally not used in modern gasoline and diesel engines containing fluoroelastomer seals despite their good antioxidant, sludge preventing, antiwear and extreme pressure properties due to their generally poor seal compatibility. Seal compatibility is of high importance in the lubricant industry, and the new GF-5 standard for lubricating oils established in October 2010 by the International Lubricants Standardization and Approval Committee (ILSAC) includes seal compatibility standards utilizing the ASTM D7216 test. The test involves immersion of a sample of the fluoroelastomer in the lubricating oil and heating at 150° C. for 336 hours. The aged fluoroelastomer sample is then tested for elongation at break, hardness, and retained tensile strength, and compared with the properties of a fresh fluoroelastomer sample.
Zinc dithiocarbamates are reaction products of a primary and/or secondary amine with carbon disulfide and a zinc source, a zinc salt to give compounds of the following structure (I):
wherein R1, R2, R3 and R4 are independently hydrogen, alkyl or arylalkyl groups. Typically zinc dithiocarbamates exist as monomers or dimers, i.e. n is 1 or 2. Typically smaller alkyl groups where R1, R2, R3 and R4 have 3 to 8 carbon atoms apiece are enough to impart oil solubility in a lubricating composition.
Fluoroelastomers (also known as Viton® a registered trademark of Dupont) are elastomers containing fluorine. They were introduced in 1957 to meet the needs of the aerospace industry for a high-performance seal elastomer. Since then, the use of Viton has expanded to many other industries, especially in the automotive, fluid power, appliance, and chemical fields.
Standard types of Viton products are designated as A, B, or F according to their relative resistance to attack by fluids and chemicals. The differences in chemical resistance are the result of different levels of fluorine in the elastomer, which is determined by the types and relative amounts of copolymerized monomers that comprise the elastomer. Typically Viton A contains 66% fluorine, Viton B 68% fluorine, and Viton F 70% fluorine.
U.S. Pat. No. 6,723,685 teaches nitrogen-containing lubricant additives are suspected, over time contributing to the deterioration of Viton seals.
U.S. Pat. No. 6,121,211 teaches a lubricating oil composition containing a metal thiocarbamate as a sludge preventive, and a Viton seal protecting amount of at least one aldehyde or epoxide or a mixture thereof. The metal thiocarbamate can be a zinc dithiocarbamate, preferably where the alkyl chain lengths R1 and R2 ranges between 3 and 5 carbon atoms. It is necessary however to have a seal protecting amount of an aldehyde and/or epoxide in the formulation.
U.S. Pat. No. 5,364,545 teaches a lubricating oil composition of a lubricating oil basestock, an organomolybdenum compound, and one or a combination of organozinc compounds consisting of zinc dithiophosphate and/or zinc dithiocarbamate along with an organic amide. However, nothing is taught about seal compatibility of these compositions.
U.S. Pat. No. 4,479,883 teaches a lubricating oil composition having particularly improved friction reducing properties which comprises an ester of a polycarboxylic acid with a glycol or glycercol and a selected metal dithiocarbamate and contains a relatively low level of phosphorus. The metal dithiocarbamate can be a zinc dithiocarbamate. Nothing is taught about seal compatibility.
U.S. Pat. No. 4,612,129 teaches sulfur-containing, oil-soluble compositions which are useful as lubricating oil additives, particularly in lubricants containing little or no phosphorus. Some of the compositions of the invention include at least one zinc dithiocarbamate wherein the alkyl groups range from between 2 and 8 carbon atoms. These lubricating compositions exhibit good nitrile seal compatibility, but no mention of fluoroelastomer seal compatibility.
U.S. Pat. No. 2,265,851 teaches metal dithiocarbamates in a lubricating oil composition in which the alkyl groups R1 and R2 together contain 8 or less carbon atoms and the metal could be zinc.
Because of the ability of zinc dithiocarbamates to perform as good antioxidants, sludge preventers, antifriction additives, and extreme pressure additives in a lubricating oil composition, it would be highly desirable to improve their fluoroelastomer seal performance.